Definitions

Combustor

Combustor

[kuhm-buhs-ter]

A combustor is a component or area of a gas turbine, ramjet or pulsejet engine where combustion takes place. It is also known as a burner or flame can depending on the design. In a gas turbine engine, the main combustor or combustion chamber is feed high pressure air by the compression system and feed the hot exhaust flows into the turbine components of the gas generator.

Combustor are designed to contain and control the burning fuel-air mixture. The combustor normally consists of three components: an outer casing that acts as a high pressure container, the combustion chamber itself which contains the flame and the fuel injection system.

Physical considerations

Within centrifugal compressor found in many small gas turbines for example in helicopter or Business jets the air reaches velocities above Mach 1. Still within the compressor it is slowed down and at the same time compressed in a first diffuser to a speed still above the flame propagation velocity of 25 m/s (Mach~0.05) and mixed with the fuel. Liquid fuel is sprayed so that it forms small droplets. Gaseous fuel is injected at a higher velocity to produce turbulences on the boundary. In some engines the fuel is heated to make it flow easily through the fuel lines and due to adiabatic compression the air has a temperature of 1000 K and the mixture has a line of sight to the flame and is heated by radiation. Hence within the following tube the fuel droplets evaporate and gaseous fuel diffuses into the air leading to a homogeneous mixture. A low swirl allows the slowest possible speed in the center of the tube and higher speeds to blow of any flames holding onto surfaces roughness or holded by surface friction. The tube blends into a second diffuser where the cross section is smoothly (to prevent flame holding) enlarged to the almost full combustor cross section and at the same time the velocity is further reduced below the flame propagation velocity (and at the same time compressed a bit). In the center a face orthogonal to the flow is left, the so called flame holder, something which was absolutely avoided upstream. Axial swirl vanes stabilize a circulation of exhaust gases through this center. Since the holder is in the center of the flame, the gas there suffers the lowest radiation cooling. Due to the centrifugal forces hot pockets in the gas and cracked molecules from intermediate steps of the reaction diffuse preferentially to the center. These ions and radicals act as a trigger to start the combustion in the already hot air fuel mixture and the premixed flame burns. In some engines the mixture is very lean and the exhaust is so cold that Nickel based alloys withstand the temperature with the help of cooling. In some engines the mixture is stoichiometric and the exhaust needs to be mixed with air. No slightly lean mixtures are used because they would produce the hottest exhaust and large number of NOx molecules. The exhaust is still too hot for the combustor walls and the guiding vanes and the turbines, so that they have to emit plenty of cooling air through fish scale porous surface or on their leading edges respectively. Sometimes an afterburner is used to burn more fuel in the lean exhaust (lean due to all the reasons mentioned above). Again in the afterburner the speed of the flow has to be very low. Often the flame-front area is enlarged by turbulence generated by air jets injected cross flow after the flame holder.

Types of combustors

There are two categories of combustors, annular and can. Can combustors look like cans and are mounted around the engine. They can be easily removed for maintenance and provide convenient plumbing for fuel. Annular combustors are more compact and embedded deep within the engine's casing. Modern Jet engines usually have annular combustors. Double annular combustors are being introduced to reduce emissions. At low throttle settings, one of the two discrete volumes in the combustor is unfuelled.

Small gas turbine engines often have a reverse flow combustor, which is a very compact design. The gas path, from high radius entry to low radius exit, is 'S' shaped.

Afterburners

Some, mainly military, turbojets and turbofans have an afterburner located in the tailpipe, to provide thrust augmentation during Take-off and Combat. On unmixed turbofans, 'afterburning' in the bypass stream is often called Plenum Chamber Burning.

Rockets

Often the combustors in a rocket engine consist of impinging jets of fluid in the main chamber. Combustion in rocket engines is performaed at much higher temperatures, as no atmospheric nitrogen is involved to cool the reaction.

Some staged combustion schemes in rocket engines combust outside of the main rocket chamber in oxidiser or fuel rich mixtures and this acts to vapourise the fuel and/or LOX so that the mixture later burns completely, quickly, stably and thoroughly in the rocket motor combustion chamber. The gases from this pre-combustion are also used to drive the turbopumps.

Scramjets

In scramjet engines the combustor inlet flow is supersonic. Only a few practical scram-jets have flown and their details are mostly classified. However, hydrogen fuel is believed to be injected into a sheltered region, below a reverse-facing step. Unlike a conventional combustor, the Mach number of the flow decreases going through the combustor.

References

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